Currently, the key element upon which EU energy policy is focused is the implementation of the so-called Third Liberalisation Package. This policy is based on a mechanism of far reaching regulation. Transmission system operators (TSOs) are at the heart of regulation of the single European energy market, because of their fundamental significance to the structure and operation of the energy sector. It is believed that the future of the European energy market and European energy security will to a large extent depend on TSOs. The study focuses on different legal issues which impact on the status of TSOs under EU law (unbundling, corporate governance, public tasks and financial management).